Process of continuously casting steel using electromagnetic field

ABSTRACT

A process for continuously casting steel slabs employing a molten steel containing an oxygen concentration of 30 ppm or less, preferably, 20 ppm or less, using a straight immersion nozzle to which an inert gas is not injected, and disposing a static magnetic field generator on the back surface of the mold for applying the strong static magnetic field to the molten steel within the mold, thereby restricting the flow of the molten steel. With this process, it is possible to prevent the nozzle blocking, and hence to obtain the steel slabs excellent in the internal and surface qualities.

TECHNICAL FIELD

The present invention relates to a process of continuously casting steelslabs for further improving the surface and internal qualities of thesteel slabs obtained by continuous casting.

BACKGROUND ART

In a process of continuously casting semi-finished products such assteel slabs used for manufacture of the broaden steel plates, arefractory material made immersion nozzle is commonly used for a moltensteel path between a tundish containing molten steel and a continuouscasting mold. The immersion nozzle is disadvantageous in that, sincealumina is liable to be deposited on the inner surface of the nozzle,particularly, in continuous casting for aluminum-killed steels, themolten steel path is narrowed with casting time, which makes itimpossible to obtain the desired flow rate of the molten steel.

In general, to prevent the deposition of alumina, an inert gas such anAr gas is supplied within the nozzle during supplying the molten steel.However, when the discharge speed of the molten steel is larger in highspeed casting with high throughput, the inert gas is trapped in the flowof the molten steel and is obstructed from being floated on the moltenpool surface within the mold, to be thus trapped in the solidifiedshell. Because of the inert gas trapped in the steel, there often occurdefects such as sliver, blistering and the like in the final products.

Also, in an immersion nozzle of a two-hole type, which includes theright and left symmetric discharge ports at the lower end portionthereof, the flow of the molten steel in the mold is liable to be madeuneven by the asymmetric blocking caused in the right and left dischargeports, thereby bringing about the lowering of the quality of theproduct. In this case, differently from the gas trap, there occur theentrapments of inclusions and mold powder due to a deflected flowgenerated by the blocking of the discharge ports of the nozzle.

The present inventors have examined the nozzle blocking in continuouscasting using a low carbon aluminum-killed steel being mainly deoxidizedby Al and containing a carbon concentration of 500 ppm or less. As aresult, it was found that the nozzle blocking was almost eliminated byadjusting the oxygen concentration in molten steel to be 30 ppm or less,preferably, 20 ppm or less, and using a pipe-like straight immersionnozzle with the leading edge being opened and served as the dischargeport for molten steel. However, such a straight nozzle isdisadvantageous in that, since the discharge flow of the molten steel isdirected downwardly of the mold, the inclusions and gas babbles inmolten steel permeate to the deep portion of the molten steel pool.

To prevent the permeation of the inclusions and the like, there has beenproposed such a technique that a static magnetic field generator forapplying a static magnetic field to the molten steel is disposed aroundthe continuous casting mold for restricting the flow of the molten steelbeing directed downwardly. For example, Japanese Patent Laid-open sho58-55157 discloses a technique of generating a direct current magneticfield in the level near the meniscus around a continuous casting mold,and of adjusting the intensity and direction thereof, therebycontrolling the permeation depth and the permeation direction of thepouring flow of the molten steel. However, in this technique, themagnetic field is applied only to the level near the meniscus, andaccordingly, the restricting force is insufficient.

The present inventors have established a technique of casting steelslabs excellent in qualities, which comprises the step of adjusting theoxygen concentration in molten steel at a lower value, and using astraight nozzle without injection of Ar gas within the nozzle, therebypreventing the nozzle blocking, while controlling the descending flow ofthe molten steel by the strong restricting force.

Further, the present inventors have found the following fact: namely,for the meniscus variation which is attributed to the flow of the moltensteel toward the meniscus generated by the effect of restricting thedescending flow of the molten steel, it is effectively restricted byapplying the static magnetic field to the meniscus portion.

A primary object of the present invention is to provide a process ofcontinuously casting steel slabs capable of obtaining the steel slabsexcellent in the surface and the internal qualities.

Another object of the present invention is to eliminate the nozzleblocking in continuous casting without using Ar gas.

A further object of the present invention is to provide a technique ofcontinuously casting the steel slabs, which comprises the steps ofapplying a suitable restricting force to the descending flow of themolten steel, and preventing the meniscus variation caused by the aboveapplication.

DISCLOSURE OF THE INVENTION

To achieve the above objects, the present invention has been made on thebasis of the above knowledge, and the technical means are as follows:namely, in the present invention, the molten steel containing an oxygenconcentration of 30 ppm or less is supplied to a continuous casting moldfrom a tundish using a straight immersion nozzle to which an inert gasis not injected, and the magnetic field is applied to the mold under thelimited condition.

The limitation preferably lies in disposing a static magnetic fieldgenerator on the back surfaces of the long side walls of the mold at theheight including the level of the discharge port of the straightimmersion nozzle; and casting the molten steel while generating a staticmagnetic field directing from one long side wall to the other long sidewall of the mold, wherein according to a discharge flow velocity <v>(m/sec) [flow rate of molten steel (m³ /sec)/nozzle sectional area (m²)]from the discharge port of the straight immersion nozzle, a relationshipbetween a magnetic flux density B (T) and an applied magnetic fieldheight range L (mm) vertically under the discharge port of the straightimmersion nozzle is set as follows:

v≦0.9 (m/sec), B×L≧25,

where B≧0.07T, L≧80 mm

v≦1.5 (m/sec), B×L≧27,

where B≧0.08T, L≧90 mm

v≦2.0 (m/sec), B×L≧30,

where B≧0.09T, L≧100 mm

v≦2.5 (m/sec), B×L≧33,

where B≧0.09T, L≧110 mm

v≦3.0 (m/see), B×L≧35,

where B≧0.1T, L≧110 mm

v≦3.8 (m/sec), B×L≧36,

where B≧0.11T, L≧120 mm

v≦4.8 (m/sec), B×L≧38,

where B≦0.12T, L≧120 mm

v≦5.5 (m/sec), B×L≧40,

where B≧0.13T, L≧130 mm

Also, the limitation preferably lies in disposing a static magneticfield generator on the back surfaces of the long side walls of the moldat the height including the level of the discharge port of the straightimmersion nozzle; disposing a gap portion, and further disposing atleast one or more stages of static magnetic field generators on thelower side than the gap portion; and casting the molten steel whilegenerating the static magnetic field directing from one long side wallto the other long side wall of the mold.

Further, the limitation preferably lies in disposing a static magneticfield generator on the back surfaces of the long side walls of the moldat the position higher than the level of the discharge port of thestraight immersion nozzle; disposing a gap portion, and furtherdisposing at least one or more stages of static magnetic fieldgenerators on the lower portion of the mold; and casting the moltensteel while generating the static magnetic field directing from one longside wall to the other long side wall of the mold.

Still further, the limitation preferably lies in applying a staticmagnetic field in the direction perpendicular to the long side surfaceof the casting only to the vicinity of the widthwise central portion ofthe casting from the back surfaces of the long side walls of the moldpositioned at the height lower than the level of the discharge port ofthe straight immersion nozzle; and applying a direct current in thedirection perpendicular to the short side surface of the casting.

Additionally, the limitation preferably lies in disposing a staticmagnetic field generator on the back surfaces of the long side walls ofthe mold at the position including the level of the discharge port ofthe straight immersion nozzle; and casting the molten steel whilegenerating the static magnetic field from one long side wall to theother long wall of the mold, and applying a direct current to thevicinity of the discharge port of the straight immersion nozzle in thedirection perpendicular to the short side surface of the casting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are schematic sectional views showing a main portionof a continuous casting apparatus including a one-stage static magneticfield generator used in Working example 1;

FIG. 2 is a graph showing the generation rate of defects in the case ofusing the one-stage static magnetic field generator in Working example1;

FIGS. 3(a) and 3(b) are sectional views showing the construction of acontinuous casting apparatus used in Working example 2;

FIG. 4 is a sectional view showing the construction of the continuouscasting apparatus used in Working example 2 with the main dimensions;

FIG. 5 is a bar graph for comparatively showing the results of Workingexample 2 in terms of the generation rate (index) of the surfacedefects;

FIGS. 6(a) and 6(b) are sectional views showing the construction of acontinuous casting apparatus used in Working examples 4 and 5;

FIG. 7 is a sectional view showing the disposition of the continuouscasting apparatus used in Working examples 4 and 5 with the maindimensions;

FIG. 8 is a bar graph for comparatively showing the results of Workingexamples 4 and 5 in terms of the generation (index) in the surfacedefects;

FIGS. 9(a) and 9(b) are schematic sectional views showing theconstruction of the main portion or a continuous casting apparatusincluding two-stage static magnetic generator used in Working example 6;

FIG. 10 is a graph showing the generation rate of the defects in thecase of using the two-stage static magnetic generator;

FIGS. 11(a) and 11(b) are schematic sectional views showing theconstruction of the main portion of a continuous casting apparatusincluding two-stage static magnetic field generator used in Workingexample 7;

FIG. 12 is a bar graph for comparatively showing the experimentalresults in the cases of using the partial static magnetic fieldgenerator (Working example 7) and the whole width static magnetic fieldgenerator (Working example 6) and no magnetic field (Comparativeexample);

FIG. 13 is a bar graph for comparatively showing the experimental resultin the cases that the static magnetic field generator is disposed at theheight including the pool surface, and that it is disposed at the heightnot including the pool surface, and further the case with no staticmagnetic field;

FIG. 14 is a bar graph for comparatively showing the experimentalresults in the cases with gas injection, and without gas injection, andfurther the case with no static magnetic field;

FIGS. 15(a) and 15(b) are sectional views of a continuous castingapparatus including a two-stage (upper and lower) static magnetic fieldgenerator used in Working examples 10 and 11;

FIGS. 16(a) and 16(b) are sectional views of a continuous castingapparatus according to the comparative example including a one-stagestatic magnetic field generator;

FIGS. 17(a) and 17(b) are sectional views of a continuous castingapparatus including a two-stage (upper and lower) static magnetic fieldgenerator provided partially in the width direction;

FIG. 18 is a graph for comparatively showing the generation rate of thesurface defects in Working examples 10 and 11 and in the conventionalexample;

FIG. 19 is a graph for comparatively showing the generation rate of thedefects in comparative examples in Working example 12;

FIG. 20 is a graph for comparatively showing the generation rate (index)of the defects in the cases of disposing the static magnetic fieldgenerator over the whole width and of disposing the magnetic fieldgenerator in the partial width as shown in Working example 13;

FIGS. 21(a) and 21(b) are sectional views showing the construction ofthe continuous casting apparatus according to Working example 14;

FIG. 22 is a bar graph for comparatively showing the results of Workingexamples 14 and 15 in terms of the generation rate of (index) of thesurface defects;

FIGS. 23(a) and 23(b) are schematic views showing Working example 16;

FIGS. 24(a) and 24(b) are explanatory views of Working example 17;

FIG. 25 is a view showing the magnetic flux density distribution in thewidth direction of the casting in Working example 17;

FIGS. 26(a), 26(b) and 26(c) are explanatory views of Working example18;

FIG. 27 is a view showing the magnetic flux density distribution in thewidth direction of the casting in Working example 18

FIGS. 28(a), 28(b) and 28(c) are schematic views of Working example 19;

FIGS. 29(a) and 29(b) are explanatory views of Example 20; and

FIGS. 30(a) and 30(b) are explanatory views of Working example 21.

BEST MODE FOR CARRYING OUT THE INVENTION

There is known the technique of disposing an electromagnet around a moldof a slub continuous casting machine, and applying a static magneticfield to molten steel in the mold, thereby controlling the flow of themolten steel by a Lorentz force caused by the mutual action between thecurrent induced in the molten steel and the magnetic field. In thistechnique, however, to prevent the flow of the molten steel dischargedfrom the immersion nozzle from permeating in the deep portion of themolten steel pool, it is insufficient to apply the static magnetic fieldonly in the vicinity of the meniscus.

FIGS. 1(a) and 1(b) show the construction of the main portion of acontinuous casting apparatus suitable for carrying out an embodiment ofthe present invention. A straight immersion nozzle 18 is suspended froma tundish into a continuous casting mold 10 constituted of a pair ofshort side walls 12, 12 and a pair of long side walls 14, 14. Thestraight immersion nozzle 18 has a pipe structure with a discharge port20 straightly opened at its lower end portion.

A static magnetic field generator 22 is disposed around the backsurfaces of the long side walls 14 and 14 of the continuous casting mold10 at the height including the vicinity of the discharge port 20 of thestraight immersion nozzle 18 and a meniscus 24, and which generates astatic magnetic field in parallel to the short side walls 12 and 12across the long side walls 14 and 14. The static magnetic field thusgenerated functions to decelerate the molten steel discharged from thestraight immersion nozzle 18 and simultaneously suppress the variationof the meniscus 24, thereby preventing the entrapment of mold power inthe molten steel.

Using the mold 10, by changing the discharge velocity <v> of the moltensteel from the straight nozzle depending on the throughput, and further,by changing the applied magnetic field intensity B and the appliedmagnetic field range L (dimension in the height direction), the defectsgenerated in the cold-rolled materials were observed. FIG. 2 shows thegeneration rate of defects effected by changing the discharge flow rate<v>, the applied magnetic field range L (mm) and the magnetic fluxdensity B (T). With respect to the cold-rolled materials obtained bychanging the magnetic-field flux and the applied magnetic field range,the generation rates of defects examined by magnetic inspection areindicated as circular marks (less than 0.45), triangular marks(0.45-0.7), and X marks (0.7 or more), with the generation rate ofdefects in the no magnetic field casting being taken as 1.

As shown in FIG. 2, as compared with no magnetic field casting,according to the present invention, the generation rate of defectsbecomes 0.045 or less in a region where the factor k =B·L obtained bythe magnetic flux density B (X-axis) and the applied magnetic fieldrange L (y-axis) is 25 or more, the applied distance L is 80 mm or more,and the magnetic flux density B is 0.07T or more.

Next, there will be described the construction as shown in FIG. 9. Inthis figure, a straight immersion nozzle 18 is used and also staticmagnetic field generators 26 and 28 are disposed in the upper and lowersides. Between the upper and lower static magnetic field generators 26and 28, a gap portion 30 being almost in no magnetic field state isprovided for equalizing the flow of the decelerated molten steel. Withthe aid of the presence of the gap portion 30, and the static magneticfield generated by the lower static magnetic field generator 28 to bedirected across the long side walls 14 and 14 in parallel to the shortside walls 12 and 12, the molten steel decelerated by the staticmagnetic field generator 26 is descended while advancing toward theshort side wall 12. As a result, it is possible to obtain thesufficiently decelerated and equalized descending flow of the moltensteel.

FIG. 10 shows the generation rate effected by changing the dischargeflow rate <v>, the magnetic flux density B and the applied magneticfield range L. In this figure, as compared with the no magnetic fieldcasting, according to the present invention, the generation rates ofdefects are indicated as circular marks (less than 0.45), triangularmarks (0.45-0.7), and X marks (0.7 or more), with the generation rate ofdefects in the cold-rolled materials obtained by the no magnetic fieldcasting being taken as 1.

As is apparent from FIG. 10, the generation rate of defects is less than0.45 in a region where the factor k=B·L obtained by the magnetic fluxdensity B and the applied magnetic field range L is 16 or more. As aresult, it becomes apparent that the applied magnetic field range ispreferable as compared with the casting with the one-stage staticmagnetic field. Thus, by applying the two-stage static magnetic field,it is possible to significantly improve the quality even when theapplied magnetic field range and the applied magnetic field intensityare small.

The above results show that, by use of the straight immersion nozzle andthe static magnetic field, it is possible to achieve the continuouscasting without nozzle blocking, and hence to improve the productivity.Further, what is more important, by eliminating the nozzle blocking, itis possible to suppress the deflected flow of the molten steel, andhence to obtain clean slabs. In particular, by specifying the magneticflux density and the applied magnetic field range, it is possible toobtain the cold-rolled materials remarkably reduced in the generationrate of defects.

Also, by applying the static magnetic field at the position includingthe molten pool surface within the continuous casting mold, it ispossible to suppress the variation of the molten pool surface. Further,by applying the static magnetic field in the vicinity of the dischargeport of the immersion nozzle, and further, by providing the gap portionand applying the static magnetic field at the lower side, it is possibleto obtain the equalized descending flow of the molten steel. This makesit possible to manufacture the further clean steel slabs without theentrapment of mold powder.

In particular, it is important to generate the static magnetic field inthe vicinity of the meniscus in a manner to cover the whole surface ofthe molten pool. For example, in the case of applying the staticmagnetic field not to the molten pool surface but only to the lowerportion of the molten pool surface, it is possible to restrict the flowunder the molten pool surface; however, it is impossible to suppress theoscillation of the molten pool surface. Accordingly, there occurs theentrapment of the mold powder on the molten pool surface due to theoscillation of the molten pool surface.

In addition, although the magnetic field achieves the important role inthe present invention, the range of the magnetic field needs to be setin the following: First, the static magnetic field must be applied tothe range containing the leading edge portion of the nozzle and thelower portion than the same. In particular, in the case that thedischarge port of the nozzle leading edge portion exists within themagnetic field, the discharge flow of the molten steel becomes themoderated descending flow by being sufficiently decelerated by themagnetic field. Next, the decelerated discharge flow becomes furtherequalized descending flow by the presence of the gap portion and thelower magnetic field, which makes it possible to obtain the castingsexcellent in the internal and surface qualities.

Further, at the lower portion where the molten steel is jetted from thedischarge port of the nozzle, it is preferable to generate the staticmagnetic field in a manner to wholly cover the continuous casting mold,as compared with the manner to partially generate the static magneticfield.

Next, in the present invention, the magnetic field by excitation may beadded. FIG. 23 shows such an example, wherein static magnetic fieldgenerating coils 60 are provided directly under a mold 10 for generatingthe static magnetic field in the direction perpendicular to the longside surface of the casting, and exciting rolls 62 for applying a directcurrent are provided in the direction perpendicular to the short sidesurface of the casting. The static magnetic field generated by thestatic magnetic field generating coils 60 are applied only to thewidthwise central portion of the casting 2 from the desired point of thelower portion than the discharge port 20 of the immersion nozzle, forexample, the position directly under the mold 10. In FIG. 23, thedirections of the magnetic field B, the current I and theelectromagnetic force F in the molten steel are shown as a chain line, adashed line, and two-dot chain line, respectively. In this case, byapplying the excitation of the static magnetic field at the lower sidethan the discharge port 20 of the immersion nozzle, it is possible toeffectively reduce the descending flow rate within the casting, therebypreventing the permeation of the inclusions and bubbles. In the staticmagnetic field exciting continuous casting process, since the dischargeflow from the nozzle usually becomes the equalized downward flow of themolten steel, the above static magnetic field excitation is applied torestrict the molten steel at the lower position than the discharge port20 of the immersion nozzle.

In the present invention, for the purpose of restricting the flow of themolten steel from the discharge port of the straight immersion nozzle,the restricting force due to excitation may be applied to the moltensteel in the vicinity of the discharge port of the nozzle. FIGS. 29(a)and 29(b) show such an example. A static magnetic field generator 82 isdisposed on the back surfaces of the long side walls 14 and 14 of acontinuous casting mold 10, and exciting terminals 84 are disposeddirectly near the discharge port of the nozzle for applying a directcurrent in the direction perpendicular to the short side surface of thecasting. In FIG. 29, the directions of the magnetic field B, the currentI and the electromagnetic force F in the molten steel are shown as achain line, dashed line and a two-dot chain line, respectively. Withthis construction, in the present invention, since the static magneticfield is generated in the molten steel within the mold in the directionperpendicular to the long side surface of the casting, andsimultaneously the direct current is applied in the directionperpendicular to the short side surface of the casting by the excitingterminals 84, it is possible to form the upward electromagnetic force Fwith respect to the casting direction, and hence to disperse thedownward flow from the nozzle. This makes it possible to suppress thepermeation of the inclusions and the babbles in the casting. Theexciting terminals may be embedded in the refractories of the straightimmersion nozzle 18.

WORKING EXAMPLE 1

The experiment was made using a two-strand type continuous castingmachine including a continuous casting apparatus as shown in FIG. 1. Lowcarbon aluminum-killed steel containing an oxygen concentration of 28-30ppm was continuously cast by three charges using a straight immersionnozzle of the present invention. The casting condition is as follows. Inaddition, the injected amount of gas for preventing the nozzle blockingwas 12N1/min.

Size of the casting mold: 220 mm in thickness

1600 mm in width

800 mm in height

Superheat of molten steel in tundish: 29°-34° C.

Throughput: 1.5 ton/min

At one strand, the casting was made under the condition of using thestraight nozzle of the present invention and applying only one-stagestatic magnetic field. At the other strand, the casting was made underthe condition of no magnetic field. FIGS. 1(a) and 1(b) are schematicviews showing the application of the one-stage static magnetic field.The specification of a static magnetic field generator 22 is as follows:

One stage static magnetic generator:

Size: 1700 mm in width, 50-650 mm (L) in height

Maximum magnetic flux density: 0.05-0.5T

By changing the discharge flow rate <v> of the molten steel depending onthe throughput, and further, by changing both the applied magnetic fieldintensity and the applied magnetic field range L, the defects caused inthe cold-rolled materials were observed. Thus, this working example wascompared with the no magnetic field casting. FIG. 2 shows a relationshipbetween the applied magnetic field range L (mm) and the magnetic fluxdensity (T), assuming that the flow rate from the nozzle discharge portis specified at 0.9 m/sec or less.

As is apparent from FIG. 2, as compared with the no magnetic fieldcasting, the generation rate of defects in this working example isimproved to be 0.45 or less in a region where the factor k=B·L obtainedby the magnetic flux density B (X-axis) and the applied magnetic fieldrange L (y-axis) is 25 or more, the applied magnetic filed range L is 80mm or less, and the magnetic flux density B is 0.07T or more. Also, forthe case that the discharge flow rate is 0.9 m/sec or more, there wereobtained the results as shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                                 Generation rate                                      Flow rate                                                                             Condition        of defect (in no                                     v (m/sec)                                                                             B × L, B (T), L (mm)                                                                     magnetic field casting:1)                            ______________________________________                                        v ≦ 1.5                                                                        B × L ≧ 27,                                                                       Less than 0.45                                               B ≦ 0.08 T, L ≧ 90 mm                                   v ≦ 2.0                                                                        B × L ≧ 30,                                                                       Less than 0.45                                               B ≧ 0.09 T, L ≧ 100 mm                                  v ≦ 2.5                                                                        B × L ≧ 33,                                                                       Less than 0.45                                               B ≧ 0.09 T, L ≧ 110 mm                                  v ≦ 3.0                                                                        B × L ≧ 35,                                                                       Less than 0.45                                               B ≧ 0.1 T, L ≧ 110 mm                                   v ≦ 3.8                                                                        B × L ≧ 36,                                                                       Less than 0.45                                               B ≧ 0.11 T, L ≧ 120 mm                                  v ≦ 4.8                                                                        B × L ≧ 38,                                                                       Less than 0.45                                               B ≧ 0.12 T, L ≧ 120 mm                                  v ≧ 5.5                                                                        B × L ≧ 40,                                                                       Less than 0.45                                               B ≧ 0.12 T, L ≧ 130 mm                                  ______________________________________                                    

WORKING EXAMPLE 2

FIGS. 3(a) and 3(b) show a continuous casting apparatus including anI-shaped static magnetic field generator 32. The I-shaped staticmagnetic field generator 32 applies the static magnetic field to therange of the flow of the molten steel discharged from a straightimmersion nozzle 18, and restricts both the downward flow of thedischarged molten steel spreading in the width direction and the flowspreading toward the meniscus forming the variation of the molten poolsurface.

By use of the straight immersion nozzle 2, the continuous casting wasmade in a manner to restrict the molten steel supplied in a continuouscasting mold 10 in the magnetic pole region of the I-shaped staticmagnetic field generator 32 disposed to the continuous casting mold 10(see FIGS. 3(a) and 3(b)). The concrete dimensions of the staticmagnetic field generator 32 are shown in FIG. 4.

Using the two-strand continuous casting machine, the molten steeladjusted by ladle refining and containing a C concentration of 360-450ppm, an Al concentration of 450-620 ppm, and an oxygen concentration of27-30 ppm was continuously cast by three charges (280t/charge) under thecondition described later. After casting, the alumina depositing stateswithin the immersion nozzles were examined. At one strand, theconventional two-hole type immersion nozzle was used. At the otherstrand, the straight immersion nozzle 18 of the present invention wasused and the above static magnetic field generator 32 was provided.

The casting condition is as follows:

Size of mold: 220 mm (short side), 1600 mm (long side)

Casting speed: 1.7 m/min

Superheat of molten steel in tundish: 25°-30° C.

Maximum magnetic flux in static magnetic field generator: 3000 gauss

As a result, in the continuous casting using the conventional two-holetype immersion nozzle into which Ar gas was injected at an injectionrate of 10N1/min for preventing the nozzle blocking, there wasrecognized an alumina depositing layer having a thickness of 10 mm atmaximum in the vicinity of the nozzle discharge port. On the other hand,in the continuous casting using the straight immersion nozzle and theI-shaped static magnetic field generator 32, in spite of no injection ofAr gas into the nozzle, it was recognized that an alumina depositinglayer was about 2 mm at maximum, and therefore, the nozzle blocking wasextremely small.

WORKING EXAMPLE 3

The molten steel containing an oxygen concentration of 15-18 ppm wasobtained by ladle refining, wherein Al power was added within the ladleon the slag on the bath surface of the molten steel having the samecomposition as in Working example 2 for reducing the FeO in the slag onthe molten steel in the ladle to be 3% or less in concentration. Theabove molten steel was continuously cast by three charges (280t/charge)under the same condition as in Working example 2. Then, the aluminadepositing states within the immersion nozzles were examined. In thisworking example, for both strands, the gas for preventing the nozzleblocking was not injected in the immersion nozzles.

As a result, in the conventional casting using the two-hole immersionnozzle, the nozzle blocking was generated at the third charge, so thatthe specified injection rate was not achieved and thus the casting speedwas reduced from 1.7 m/min to 1.2 m/min. On the other hand, in thecontinuous casting using the straight immersion nozzle, the castingspeed was not reduced. After the casting, the inner surface of therecovered straight immersion nozzle was observed, which gave the resultthat the alumina was deposited thereon only to a thickness of about 1-2mm.

In addition, the experiment using the straight immersion nozzle withoutthe static magnetic field was made separately. In the above, the jet ofthe high temperature molten steel discharged from the leading edge ofthe nozzle was made to strongly flow downwardly in the verticaldirection to wash the solidified shell, thereby obstructing the progressof solidification of the portion. Thus, the so-called breakout wasgenerated, and thereby the casting was made impossible. On the contrary,in Working examples 2 and 3 using the straight nozzle with the staticmagnetic field, as described above, the stable casting was madepossible.

The continuous casting slabs obtained in Working examples 2 and 3 werehot-rolled and cold-rolled to a thickness of 0.7 mm. The cold-rolledsteel plates thus obtained were examined for the generation rate of thesurface defects (total of blistering defects and sliver defects). Theresults are shown in FIG. 5.

As is apparent from FIG. 5, it is revealed that the generation rate ofthe surface defects is extremely small in the continuous castingaccording to the present invention. The reason for this is as follows:namely, by the application of the static magnetic field to thecontinuous casting mold, the pouring flow of the molten steel isprevented from permeating to the deep portion of the crater; and theflow of the molten steel at the meniscus is restricted, therebyeliminating the entrapment of the mold powder. Also, the reason why theresult obtained from the suitable example in Working example 3 is morepreferable than that in Working example 2 is considered as follows:namely, the oxygen concentration in the molten steel is low and the Argas injection as a main cause of generating the blistering defects isnot performed. In addition, even in the comparative example in Workingexample 3, the fairly preferable result is obtained; however, since thegas for preventing the nozzle blocking is not injected in the nozzle,the nozzle blocking is generated, thereby making it impossible to obtainthe desired casting speed, which brings about the problem inproductivity.

WORKING EXAMPLE 4

By use of a two-strand type continuous casting machine including aT-shaped static magnetic field generator as shown in FIG. 6, the moltensteel adjusted by ladle refining and containing a C concentration of380-500 ppm, an Al concentration of 450-550 ppm and an oxygenconcentration of 25-28 ppm, was continuously cast by three charges(300t/charge) under the condition described later. After casting, thealumina depositing states within the straight immersion nozzles wereexamined.

At one strand, a straight immersion nozzle 18 was used and a T-shapedstatic magnetic field generator 34 was disposed in such a dimensionalrelation as shown in FIG. 7. At the other strand, the conventionaltwo-hole type immersion nozzle was used.

The casting condition was as follows:

Size of mold: 215 mm (short side), 1600 mm (long side)

Casting speed: 1.6 m/min

Superheat of molten steel in tundish: 20°-25° C.

Maximum magnetic flux in static magnetic field generator: 3200 gauss

As a result, in the continuous casting using the conventional two-holetype immersion nozzle into Which Ar gas was injected at an injectionrate 10N1/min for preventing the nozzle blocking, there was recognizedan alumina depositing layer having a thickness of 10 mm at maximum inthe vicinity of the nozzle discharge port. On the other hand, in thecontinuous casting using the straight immersion nozzle with the staticmagnetic field, in spite of no injection of Ar gas into the nozzle, itwas recognized that an alumina depositing layer was about 2 mm atmaximum, and therefore, the nozzle blocking was extremely small.

WORKING EXAMPLE 5

The molten steel containing an oxygen concentration of 12-18 ppm wasobtained by ladle refining, wherein Al power was added within the ladleon the slag on the bath surface of the molten steel having the samecomposition as in Working example 4 for reducing the FeO in the slag onthe molten steel in the ladle to be 2% or less in concentration. Theabove molten steel was continuously cast by three charges (300t/charge)under the same condition as in Working example 4. Thus, the aluminadepositing states within the immersion nozzles were examined. In thisworking example, for both strands, the gas for preventing the nozzleblocking was not injected in the immersion nozzles.

As a result, in the conventional casting using the two-hole immersionnozzle, the nozzle blocking was generated at the third charge, so thatthe specified injection rate was not achieved and thus the casting speedwas reduced from 1.6 m/min to 1.1 m/min. On the other hand, in thecontinuous casting according to this working example, the casting speedwas not reduced. After the casting, the inner surface of the recoveredstraight immersion nozzle 18 was observed, which gave the result thatthe alumina was deposited thereon only to a thickness of about 1-2 mm.

In addition, the experiment using the straight immersion nozzle 18without the static magnetic field was made separately.

In the above, the jet of the high temperature molten steel dischargedfrom the leading edge of the nozzle was made to strongly flow downwardlyin the vertical direction to wash the solidified shell, therebyobstructing the progress of solidification of the portion. Thus, theso-called breakout was generated, and thereby the casting was madeimpossible. On the contrary, in Working examples 4 and 5 using thestatic magnetic field 34, as described above, the stable casting wasmade possible.

The continuous casting slabs obtained in Working examples 4 and 5 werehot-rolled and cold-rolled to a thickness of 0.8 mm. The cold-rolledsteel plates thus obtained were examined for the generation rate of thesurface defects (total of blistering defects and sliver defects). Theresults are shown in FIG. 8.

As is apparent from FIG. 8, it is revealed that the generation rate ofthe surface defects is extremely small in the suitable example. Thereason for this is as follows: namely, by the application of the staticmagnetic field to the continuous casting mold, the pouring flow of themolten steel is prevented from permeating to the deep portion of thecrater; and the flow of the molten steel at the meniscus is restricted,thereby eliminating the entrapment of the mold powder. Also, the reasonwhy the result obtained from the suitable example in Working example 5is more preferable than that in Working example 4 is considered as thefollows: namely, the oxygen concentration in the molten steel is low andthe Ar gas injection as a main cause of generating the blisteringdefects is not performed. In addition, even in the comparative examplein Working example 5, the fairly preferable result is obtained; however,since the gas for preventing the nozzle blocking is not injected in thenozzle, the nozzle blocking is generated, thereby making it impossibleto obtain the desired casting speed, which brings about the problem inproductivity.

WORKING EXAMPLE 6

Next, as illusted in FIG. 9, the casting experiments were made asfollows: At one strand, a straight injection nozzle 18 was used andstatic magnetic field generators 26 and 28 were disposed on the upperand lower sides for applying the upper and lower static magnetic fieldsin two stages. At the other strand, the conventional two-hole typeimmersion nozzle was used as a comparative example. In the casting, thegas for preventing the nozzle blocking was injected at an injection rateof 10N1/min in both the above strands. The other casting condition wasthe same as in Working example 1.

The specifications of the upper and lower static magnetic fieldgenerators are as follows:

Upper static magnetic field generator:

Size: 1700 mm in width, 50-320 mm (L₁) in height

Maximum magnetic flux density: 0.05-0.6T

Interval between magnetic poles: 300 mm (from lower end of upper staticmagnetic field generator to upper end of lower static magnetic fieldgenerator)

Lower static magnetic field generator:

Size: 1700 mm in width, 50-320 mm (L₂) in height

Maximum magnetic flux density: 0.05-0.5T

Whole range of magnetic poles: L₁ +L₂ =100-640 mm

Assuming that the discharge flow rate is less than 0.9 m/sec, bychanging the discharge flow rate <v>, the magnetic flux density B andthe applied magnetic field range L, the generation rates of defects wereobtained. Ther results are shown in FIG. 10. In this figure, thegeneration rates of defects in this working example are indicated ascircular marks (less than 0.45), triangular marks (0.45-0.7) and X marks(0.7 or more), with the generation rate of defects in the cold-rolledmaterial obtained by the no magnetic field casting being taken as 1.

As is apparent From FIG. 10, the generation rate of defects in thisexample becomes less than 0.45 in a region where the factor k=B·Lobtained by the magnetic flux density B (X-axis) and the appliedmagnetic field range L (y-axis) is 16 or more. As a result, it becomesclear that the applied magnetic field range is more preferable ascompared with the case using the one-stage magnetic field.

Even in the case that the discharge flow rate becomes larger than thevalue of 0.9 m/sec, similarly, the flow of the molten steel was able tobe controlled by applying the two-stage static magnetic field. Theresults are shown in Table 2. As is apparent from Table 2, by applyingthe two-stage static magnetic field, it is possible to extremely improvethe quality as compared with the no magnetic casting even when theapplied magnetic field range and the applied magnetic field intensityare small.

                  TABLE 2                                                         ______________________________________                                                                 Generation rate                                      Flow rate                                                                             Condition        of defect (in no                                     v (m/sec)                                                                             B × L, B (T), L (mm)                                                                     magnetic field casting:1)                            ______________________________________                                        v ≦ 1.5                                                                        B × L ≧ 18,                                                                       Less than 0.45                                               B ≧ 0.07 T, L ≧ 70 mm                                   v ≦ 2.0                                                                        B × L ≧ 19,                                                                       Less than 0.45                                               B ≧ 0.08 T, L ≧ 70 mm                                   v ≦ 2.5                                                                        B × L ≧ 20,                                                                       Less than 0.45                                               B ≧ 0.09 T, L ≧ 80 mm                                   v ≦ 3.0                                                                        B × L ≧ 21,                                                                       Less than 0.45                                               B ≧ 0.1 T, L ≧ 90 mm                                    v ≦ 4.0                                                                        B × L ≧ 22,                                                                       Less than 0.45                                               B ≧ 0.11 T, L ≧ 100 mm                                  v ≦ 5.0                                                                        B × L ≧ 24,                                                                       Less than 0.45                                               B ≧ 0.12 T, L ≧ 100 mm                                  v ≦ 6.0                                                                        B × L ≧ 40,                                                                       Less than 0.45                                               B ≧ 0.13 T, L ≧ 110 mm                                  ______________________________________                                    

WORKING EXAMPLE 7

The experiments were made under the same condition as in Working example6 for comparing the method of applying the magnetic field to the wholewidth of the mold as shown in FIG. 11(b), with the method of applyingthe magnetic field to the partial width of the mold as shown in FIG.11(a). Further, for comparison, casting was made by the conventionalmanner. On the basis of the results of the above experiments, thedifference according to the method of applying the magnetic field wasexamined. By use of a two-strand continuous casting machine, a lowcarbon aluminum-killed steel containing an oxygen concentration of 20-24ppm was continuously cast. In both the strands, the gas for preventingthe nozzle blocking was injected at an injection rate of 10N1/min. Thecasting condition is as follows:

Size of casting mold: 220 mm in thickness

1600 mm in width

800 mm in height

Superheat of molten steel in tundish: 28°-33° C.

Casting speed: 3.0 m/min

The specification of the partial static magnetic field generator is asfollows:

Upper static magnetic field generator:

Size: 800 mm in width, 300 mm in height

Maximum magnetic flux density: 0.31T

Interval of magnetic poles: 300 mm (from lower end of upper magneticfield generator to upper end of lower static magnetic field generator)

Lower static magnetic field generator:

Size: 800 mm in width, 300 mm in height

Maximum magnetic flux density: 0.31T

Also, the specification of the whole static magnetic field generator isas follows:

Upper static magnetic field generator:

Size: 1700 mm in width, 300 mm in height

Maximum magnetic flux density: 0.31T

Interval of magnetic poles: 300 mm (from lower end of upper magneticfield generator to upper end of lower static magnetic field generator)

Lower static magnetic field generator:

Size: 1700 mm in width, 300 mm in height

Maximum magnetic flux density: 0.31T

The results are shown in FIG. 12. As is apparent from FIG. 12, thegeneration rate of defects becomes extremely smaller in the case ofapplying the static magnetic field in the width of 1700 mm. Accordingly,it becomes clear that the application of the static magnetic field overthe whole width of the mold is effective to improve the quality.

WORKING EXAMPLE 8

The experiments were made according to the casting process using thestraight nozzle of the present invention and applying the staticmagnetic fields in multi-stage with the gap portion, for comparing thecase that the upper stage magnetic field included the meniscus and thevicinity of the discharge port of the immersion nozzle, with the casethat it included only the discharge port of the immersion nozzle. Theexperiments were made using a two-strand continuous casting machine,under the following condition:

Size of mold: 220 mm in thickness

1600 mm in width

800 mm in height

Superheat of molten steel in tundish: 24°-30° C.

Casting speed: 1.9 m/min

A low carbon aluminum-killed steel containing an oxygen concentration of28 ppm was continuously cast by three charges. The gas for preventingthe nozzle blocking was injected at an injection rate of 12N1/min.

The specification of the multi-stage type static magnetic fieldgenerator is as follows:

Upper static magnetic field generator:

Size: 1700 mm in width, 250 mm in height

Maximum magnetic flux density: 0.27T

Interval of magnetic poles: 300 mm (from lower end of upper magneticfield generator to upper end of lower static magnetic field generator)

Lower static magnetic field generator:

Size: 1700 mm in width, 250 mm in height

Maximum magnetic flux density: 0.27T

In this case, the comparative experiments were made between the casethat the upper magnetic field generator is disposed at the heightincluding the molten pool surface, and the case that it is disposed atthe height not including the molten pool surface. Further, forcomparison, the conventional casting was made. The generation rates ofdefects in this working example were standardized, with the generationrate of defects in the conventional casting being taken as 1. As isapparent from FIG. 13, according to the present invention, thegeneration rate of defects is smaller in the case that the staticmagnetic field is disposed at the height including the molten poolsurface.

WORKING EXAMPLE 9

To examine the blocking state of the nozzle in casting without injectionof the gas for preventing the nozzle blocking, the experiments were madeunder the following condition. A low carbon aluminum-killed steeladjusted by ladle refining to be reduced in an oxygen concentration of15-20 ppm was continuously cast.

Size of casting mold: 220 mm in thickness

1600 mm in width

800 mm in height

Superheat of molten steel in tundish: 28°-33° C.

Casting speed: 2.2 m/min

In the experiments required for the gas injection in both theconventional casting and the magnetic field applying casting, the gasfor preventing the nozzle blocking was injected at an injection rate of12N1/min.

The specification of the multi-stage type static magnetic fieldgenerator is as follows:

Upper static magnetic field generator:

Size: 1700 mm in width, 270 mm in height

Maximum magnetic flux density: 0.29T

Interval of magnetic poles: 300 mm (from lower end of upper magneticfield generator to upper end of lower static magnetic field generator)

Lower static magnetic field generator:

Size: 1700 mm in width, 270 mm in height

Maximum magnetic flux density: 0.29T

In the casting using the straight nozzle, even when the gas injectionfrom the nozzle was not performed, there was recognized the depositedinclusions in a thickness of about 1 mm within the nozzle after beingused by three charges, which gave the result almost equivalent to thatobtained in the case of performing the gas injection.

FIG. 14 shows the generation rate of defects of this working example. Asis apparent from FIG. 14, the generation rate of defects is reduced inthe case without the gas injection. Accordingly, by performing thecasting without the gas injection, it is possible to obtain the steelplate excellent in cleanliness. Incidentally, even in the case ofperforming the gas injection, the generation rate of defects issufficiently reduced.

WORKING EXAMPLE 10

The continuous casting was made using a continuous casting apparatus asshown in FIGS. 15(a) and 15(b). As shown in FIGS. 15(a) and 15(b), therewas used a straight immersion nozzle 18 having a straight discharge port20 being opened at the leading edge of the nozzle main body. Further,upper and lower static magnetic fields 42 and 44 were applied.

The upper static magnetic field generator 42 disposed to a continuouscasting mold 10 makes quiet the surface of the molten steel suppliedwithin the mold 10 while restricting the molten steel in the magneticpole range, and further, equalizes the descending flow of the moltensteel at a gap portion 46. Also, the lower static magnetic fieldgenerator 44 restricts the molten steel during casting.

By use of a two-strand continuous casting machine, a low carbonaluminum-killed steel containing an oxygen concentration of 20-30 ppmwas continuously cast by three charges using the immersion nozzle of thepresent invention. The casting condition is as follows:

Size of mold: 200 mm in thickness

1500 mm in width

800 mm in height

Superheat of molten steel in tundish: about 30° C.

Casting speed: 2.0 m/min

At one strand, a straight immersion nozzle 18 was used and the upper andlower static magnetic fields 42 and 44 were applied. At the otherstrand, the conventional two-hole type immersion nozzle was used. Also,in both the strands, the gas for preventing the nozzle blocking wasinjected at an injection rate of 10N1/min. The specification of thestatic magnetic field generator is as follows:

Upper static magnetic field generator:

Size: 1700 mm in width, 300 mm (L₁) in height

Maximum magnetic flux density: 0.4T

Lower static magnetic field generator:

Size: 1700 mm in width, 300 mm (L₂) in height

Maximum magnetic flux density: 0.4T

Interval of magnetic poles: 300 mm (from lower end of upper magneticfield generator to upper end of lower static magnetic field generator)

Whole range of magnetic poles: L₁ +L₂ =600 mm

As a result, in the continuous casting using the conventional two-holetype immersion nozzle, there was recognized the alumina depositing layerhaving a thickness of 12 mm at maximum in the vicinity of the dischargeport of the nozzle. On the contrary, in the continuous casting using thestraight immersion nozzle with the static magnetic field, there wasrecognized the alumina depositing layer having a thickness of 1.0 mm onaverage at the opening portion of the discharge port. Therefore, itbecomes apparent that the nozzle blocking is extremely small in thisworking example.

WORKING EXAMPLE 11

The experiments were made under the same condition as in Working example11, except that the gas injection was not performed in both the strands.The casting speed was 2.0 m/min, which was the same as in Workingexample 10. Also, before the experiments, the molten steel was adjustedby ladle refining to be reduced in an oxygen concentration of 15-20 ppm.As a result, in the casting using the two-hole type immersion nozzle,the opening degree of a sliding nozzle was started to be increased atthe second charge, thereby making difficult the essential flow control,and in the period near the end of the pouring process at the thirdcharge, the desired pouring speed was not achieved due to the nozzleblocking, thereby reducing the casting speed. On the contrary, in thecasting using the straight immersion nozzle 18 of the present inventionand applying the static magnetic fields 42 and 44, the nozzle blockingwas not generated and thus the pouring speed was not reduced, as aresult of which the casting speed was not reduced.

Both the nozzles were recovered after the experiments, and were comparedwith each other in the blocking state of the nozzle. In the straightimmersion nozzle, there was recognized the depositing alumina having athickness of 1.0 mm or less on average. On the other hand, in thetwo-hole type immersion nozzle, there was generated the alumina depositsat the discharge port, and further, the depositing states in the twoholes of the immersion nozzle were not uniform, which makes unequal theright and left discharged flows to each other.

FIG. 18 shows the results obtained from Working examples 10 and 11. InFIG. 18, there are shown the defects on average measured by magneticinspection per unit area of the cold-rolled steel plates which areobtained by hot-rolling and cod-rolling the slabs continuously cast.Further, after the measurement by magnetic inspection, there wasexamined the causes of the defects. As a result, it was revealed thatthe defects due to gas, the defects due to inclusions and the defectsdue to powder were at stake. With the generation rate of surface defectsin the cold-rolled plate obtained in Working example 10 being taken as1, the other generation rates of defects were indicated.

FIG. 18 shows the generation rate of defects in Working examples 10 and11 in which the casting process of the present invention is comparedwith the conventional casting. As is apparent from this figure, in thepresent invention, the internal defects of the slab is remarkablyreduced as compared with the conventional casting. As shown in Workingexample 11 of FIG. 18, particularly, in the case that the cleanliness ofthe molten steel is high, the nozzle blocking is eliminated, andfurther, the blowhole defects are never generated because of no gasinjection, thus obtaining the preferable results.

WORKING EXAMPLE 12

The experiments were made for comparing a case of applying the two-stagestatic magnetic field including a gap portion, with a case of applyingthe one-stage static magnetic field. In either experiment, the straightimmersion nozzle was used. The casting condition is as follows. Inaddition, the injected amount of the gas for preventing the nozzleblocking was specified to be 15N1/min in a total amount from the uppernozzle and the sliding nozzle.

Size of casting mold: 200 mm in thickness

1500 mm in width

800 mm in height

Superheat of molten steel in tundish: about 30° C.

Casting speed: 1.9 m/min

In the above, a low carbon aluminum-killed steel containing an oxygenconcentration of 28 ppm was continuously cast by three charges.

FIG. 19 shows the comparison between the experimental result obtained inthe case that the two-stage static magnetic field is applied and thenozzle discharge port exists in the upper static magnetic field as shownin FIG. 15, and the experimental result obtained in the case of applyingthe one-stage static magnetic field as shown in FIG. 16 (comparativeexample). The specifications of respective static magnetic fieldgenerators are as follows:

Two-stage static magnetic field generator

Upper static magnetic field generator:

Size: 1700 mm in width, 300 mm (L₁) in height

Maximum magnetic flux density: 0.4T

Lower static magnetic field generator:

Size: 1700 mm in width, 300 mm (L₂) in height

Maximum magnetic flux density: 0.4T

Interval of magnetic poles: 300 mm (from lower end of upper magneticfield generator to upper end of lower static magnetic field generator)

Whole range of magnetic poles: L₁ +L₂ =600 mm

One-stage static magnetic field generator

Size: 1700 mm in width, 600 mm (L) in height

Maximum magnetic flux density: 0.4T

FIG. 19 shows the generation rate of defects measured by magneticinspecting device. With the generation rate of defects in theconventional casting being taken as 1, the generation rates of defectsin the working example and the comparative example are shown. As aresult, it becomes apparent that the generation rate of the defects inthe present invention is small.

The reason why the generation rate of defects is higher in thecomparative example as compared with the present invention is that,since there is no gap in the applied magnetic field, the flow of themolten steel is difficult to be diffused as compared with the presentinvention, so that the discharge flow is difficult to be made theuniform descending flow. Accordingly, the inclusions and babbles aremade to run along the discharge flow and to be thus trapped by the shelldirectly under the nozzle. However, the above comparison is made underthe condition of applying the magnetic field, and accordingly, thecomparative example is remarkably improved as compared with theconventional example with no magnetic field. The reason for this is thatthe variation in the molten pool surface is suppressed by the appliedstatic magnetic field in the present invention and the comparativeexample.

Further, in the present invention, the discharge flow is not onlydecelerated but also diffused at the gap portion provided between theupper and lower static magnetic fields, and is made to be the uniformdescending flow by the lower static magnetic field.

WORKING EXAMPLE 13

The experiments were made for comparing a case of applying the staticmagnetic field in the whole width range of the mold, with a case ofapplying the static magnetic field in a partial width range of the mold.A low carbon aluminum-killed steel containing an oxygen concentration of20-24 ppm was continuously cast using a two-strand continuous castingmachine. In both the strands, the gas for preventing the nozzle blockingwas injected at an injection rate of 10N1/min.

The casting condition is as follows:

Size of mold: 200 mm in thickness

1500 mm in width

800 mm in height

Superheat of molten steel in tundish: about 30° C.

Casting speed: 2.2 m/min

FIG. 17 shows the two-stage static magnetic field generator forpartially applying the static magnetic field. The specification of thestatic magnetic field generator is as follows:

Upper static magnetic field generator:

Size: 800 mm in width, 300 mm (L₁) in height

Maximum magnetic flux density: 0.4T

Interval of magnetic poles: 300 mm (from lower end of upper magneticfield generator to upper end of lower static magnetic field generator)

Lower static magnetic field generator:

Size: 800 mm in width, 300 mm (L₂) in height

Maximum magnetic flux density: 0.4T

The experiment was made by disposing the above two-stage static magneticfield generator at one strand. Also, for comparison, another experimentwas made at the other strand under the same condition as in Workingexample 10. The results are shown in FIG. 20. As is apparent from FIG.20, it is preferable to apply the static magnetic field in a width rangeof 1700 mm. However, even in the case of partially applying the staticmagnetic field, it is more preferable as compared with the conventionalcasting process.

WORKING EXAMPLE 14

The continuous casting was performed using a continuous castingapparatus as shown in FIGS. 21(a) and 21(b). By use of a straightimmersion nozzle 18 having a straight discharge port 20 being opened atthe leading edge of the nozzle main body, the continuous casting wasmade by restricting the molten steel supplied into a continuous castingmold 10 from the nozzle in the magnetic pole range of a static magneticfield generator 58 disposed on the lower portion of the continuouscasting mold 10 (see FIGS. 21(a) and 21(b)).

As a result, there is eliminated the inconvenience of the nozzleblocking caused by the alumina deposition, and accordingly, even whenthe molten steel is poured in the mold at the desired speed, theinclusions doe not permeate in the deep portion of the molten steel.Also, even when the flow of the molten steel in the meniscus directionby the restricting effect, the flow of the molten steel is restricted bythe static magnetic field from the static magnetic field generator 56disposed at the position corresponding to the meniscus portion, whichmakes it possible to prevent the entrapment of the mold powder on thebath surface.

WORKING EXAMPLE 15

By use of a two-strand continuous casting machine, the molten steeladjusted by ladle refining and containing a C concentration of 400-550ppm, an Al concentration of 400-570 ppm, and an oxygen concentration or23-29 ppm was continuously cast by three charges (285t/charge) under thecondition described later. After the casting, the alumina depositingstates within the straight immersion nozzles were examined. As shown inFIG. 21, a lower static magnetic field generator 58 was disposed in sucha manner that the upper end thereof was held at the position lower thanthe lowermost end portion of the immersion nozzle by 100 mm, and thelower end thereof was held at the position lower than the lowermost endportion of the discharge port by 600 mm. An upper static magnetic fieldgenerator 56 was disposed in such a manner that the upper end thereofwas held at the position higher than a molten steel meniscus 24 by 100mm, and the lower end thereof was held at the position lower than themeniscus 24 by 200 mm. At one strand, the conventional two-hole typeimmersion nozzle was used. At the other strand, the straight immersionnozzle 18 was used and the static magnetic field generators 56 and 58were disposed.

The casting condition is as follows:

Size of mold: 240 mm (short side wall)

1600 mm (long side wall)

Casting speed: 1. 65 m/min

Superheat of molten steel in tundish: about 25°-30° C.

The specification of the static magnetic field generator is as follows:

Upper static magnetic field generator:

Size: 1700 mm in width, 300 mm in length

Maximum magnetic flux: about 3150 gauss

Lower static magnetic field generator:

Size: 1700 mm in width, 500 mm in length

Maximum magnetic flux: about 3150 gauss

In the continuous casting using the conventional two-hole type immersionnozzle to which the gas for preventing the nozzle blocking was injectedat an injection rate of 10N1/min, there was recognized an aluminadepositing layer having a thickness of 10 mm at maximum in the vicinityof the nozzle discharge port. On the contrary, in the continuous castingusing the straight immersion nozzle with the static magnetic field, indespite of no injection of Ar gas in the nozzle, it was revealed thatthe alumina depositing layer was generated within the nozzle to athickness of about 2 mm at maximum, and accordingly, the nozzle blockingwas extremely small.

The molten steel containing an oxygen concentration of 12-16 ppm wasobtained by ladle refining, wherein Al power was added within the ladleon the slag on the bath surface of the molten steel having the samecomposition as in Working example 14 for reducing the FeO in the slag onthe molten steel in the ladle to be 2.3% or less in concentration. Theabove molten steel was continuously cast by three charges (285t/charge)under the same condition as in Working example 14. Thus, the aluminadepositing states within the immersion nozzles were examined. In thisworking example, for both strands, the gas for preventing the nozzleblocking was not injected in the immersion nozzles.

As a result, in the conventional casting using the two-hole immersionnozzle, the nozzle blocking was generated at the third charge, so thatthe specified injection rate was not achieved and thus the casting speedwas reduced from 1.65 m/min to 1.0 m/min. On the other hand, in thecontinuous casting using the straight immersion nozzle with the staticmagnetic field, the casting speed was not reduced. After the casting,the inner surface of the recovered straight immersion nozzle wasobserved, which gave the result that the alumina was deposited thereononly to a thickness of about 1-2 mm.

In addition, the experiment using the straight immersion nozzle withoutthe static magnetic field, and the experiment using only lower staticmagnetic field generator were made separately. In the former experiment,the jet of the high temperature molten steel discharged from the leadingedge of the nozzle was made to strongly flow downwardly in the verticaldirection to wash the solidified shell, thereby obstructing the progressof solidification of the portion. Thus, the so-called breakout wasgenerated, and thereby the casting was made impossible. Also, in thelatter experiment, the variation in the molten pool surface becomeslarger thereby making impossible the stable casting. Further, as aresult of observation for the surface of the cold-rolled steel plateobtained by rolling the slab cast in the latter experiment, there wasrecognized the lot of entrapment of the mold powder. On the contrary, inWorking examples 14 and 15, as described above, the stable casting waspossible by the application of the upper and lower static magneticfields.

The continuous casting slabs obtained in Working examples 14 and 15 werehot-rolled and cold-rolled to a thickness of 1.0 mm. The cold-rolledsteel plates thus obtained were examined for the generation rate of thesurface defects (total of blistering defects and sliver defects). Theresults are shown in FIG. 22.

As is apparent from FIG. 22, it is revealed that the generation rate ofthe surface defects is extremely small in the continuous casting usingthe straight immersion nozzle with the static magnetic field. The reasonfor this is as follows: namely, by the application of the staticmagnetic field to the continuous casting mold, the pouring flow of themolten steel is prevented from permeating to the deep portion of thecrater; and the flow of the molten steel at the meniscus portion isrestricted thereby eliminating the entrapment of the mold powder. Also,the reason why the result obtained from the suitable example in Workingexample 15 is more preferable than that in Working example 14 isconsidered as follows: namely, the oxygen concentration in the moltensteel is low and the Ar gas injection as a main cause of generating theblistering defects is not performed. In addition, even in thecomparative example in Working example 15, the fairly preferable resultis obtained; however, since the gas for preventing the nozzle blockingis not injected in the nozzle, the nozzle blocking is generated, therebymaking it impossible to obtain the desired casting speed, which bringsabout the problem in productivity.

WORKING EXAMPLE 16

FIG. 23 is a view for explaining the construction of this workingexample. Directly under a mold 10, there are provided static magneticfield generating coils 60 for generating a static magnetic field in thedirection perpendicular to the long side surface of the casting, andexciting rolls 62 for applying a direct current in the directionperpendicular to the short side surface of the casting. The staticmagnetic field generated at the static magnetic field generating coil 60is applied to a widthwise central portion of the casting 2 from asuitable point under the discharge port 20 of the immersion nozzle, forexample, at the position directly under the mold 10. In FIG. 23, thedirections of the magnetic field B, the current I, and theelectromagnetic force F in the molten steel are shown in a chain line, adashed line, and two-dot chain line, respectively.

In addition, in the above construction as shown in FIG. 23, there areshown the static magnetic field generating coils 60 and the excitingrolls 62 set in one-stage in the casting direction under the level ofthe immersion nozzle discharge port 20; however, the same constructionsmay be set in two or more stages in the casting direction.

In this experimental example, by applying the static magnetic field toonly the position near the widthwise central portion of the castingunder the immersion nozzle discharge port 20, it is possible toeffectively reduce the descending flow rate within the casting, andhence to prevent the permeation of the inclusions and babbles.

In the continuous casting using the straight immersion nozzle 18 withthe static magnetic field excitation, the discharge flow of the moltensteel from the nozzle is usually made to the uniform descending flow, sothat the above static magnetic field excitation may be applied only inthe vicinity of the widthwise central portion of the casting 2 at theposition under the immersion nozzle discharge port 20, to thus restrictthe flow of the molten steel.

Extremely low carbon aluminum-killed steel (C=10-20 ppm), which wasobtained by RH treatment after blowing in a converter, was continuouslycast by six strands (285t/strand) at a throughput of 6.0t/(min·strand)under the following condition.

Size of slab: 215 mm (t)×1500 mm (W)

Type of continuous casting machine: vertical bending continuous castingmachine, two strand, vertical portion (2 m)

Superheat of molten steel in tundish: 15°-20° C.

Immersion depth of nozzle: 250 mm (distance between meniscus and nozzlejetting port)

Oxygen concentration of molten steel in tundish: 12-15 ppm

Length of mold: 900 mm

Distance between meniscus and lower end of mold: 800 mm

Slabs were continuously cast according to respective casting processesdescribed later, and then hot-rolled and cold-rolled to a thickness of0.7 mm. The cold-rolled steel plates thus obtained were examined in aninspecting line, and were compared with each other in the generationrate of sliver and blistering defects caused by steel-making. As aresult, according to the present invention, it is possible to extremelyreduce the generation rate of defects as compared with the conventionalcasting.

COMPARATIVE EXAMPLE 16-1

Immersion nozzle: two-hole nozzle, no static magnetic field

Flow rate of Ar gas injected in immersion nozzle: 15N1/min

Generation rate of internal and surface defects of cold-rolled steelplate: 3.6%

COMPARATIVE EXAMPLE 16-2

Immersion nozzle: two-hole nozzle

Intensity of static magnetic field: 0.35T

Flow rate of Ar gas injected in immersion nozzle: 15N1/min

Generation rate of internal and surface defects of cold-rolled steelplate: 2.8%

WORKING EXAMPLE 16-1

Immersion nozzle: single straight nozzle discharge port (80 mmφ)

Setting position of static magnetic field: one piece, being set atposition apart from meniscus by 900-1050 mm to apply static magneticfield to widthwise central portion of casting

Intensity of static magnetic field: 0.35T

Applied current: 3500A (DC)

Injection of gas into immersion nozzle: not performed

Generation rate of internal and surface defects of cold-rolled steelplate: 0.3%

WORKING EXAMPLE 17

FIG. 24 is a view for explaining the construction of this workingexample 17. Directly under a mold 10, there are provided static magneticfield generating coils 64 for generating a static magnetic field in thedirection perpendicular to the long side surface of the casting, andexciting rolls 66 for applying a direct current in the directionperpendicular to the short side surface of the casting. The staticmagnetic field generated at the static magnetic field generating coils60 is applied to the whole width of the casting 2 from a suitable pointunder the discharge port 20 of the immersion nozzle, for example, at theposition directly under the mold 10. In FIG. 24, the directions of themagnetic field B, the current I, and the electromagnetic force F in themolten steel are shown in a chain line, a dashed line, and two-dot chainline, respectively.

Extremely low carbon aluminum-killed steel (C=15-25 ppm), which wasobtained by RH treatment after blowing in a converter, was continuouslycast by six strands (280t/strand) at a throughput of 5.5t/(min·strand)under the following condition.

Size of slab: 220 mm (t)×1500 mm(W)

Type of continuous casting machine: vertical bending continuous castingmachine, two strands, vertical portion (3 m)

Superheat of molten steel in tundish=15°-25° C.

Immersion depth of nozzle: 300 mm (distance between meniscus and nozzlejetting port)

Oxygen concentration of molten steel in tundish: 13-18 ppm

Length of mold: 900 mm

Distance between meniscus and lower end of mold: 800 mm

Slabs were continuously cast according to respective casting processesdescribed later, and then hot-rolled and cold-rolled to a thickness of0.8 mm. The cold-rolled steel plates thus obtained were examined in aninspecting line, and were compared with each other in the generationrate of sliver and blistering defects caused by steel-making. As aresult, according to the present invention, it is possible to extremelyreduce the generation rate of defects as compared with the conventionalcasting.

COMPARATIVE EXAMPLE 17-1

Immersion nozzle: two-hole nozzle

Flow rate of Ar gas injected in immersion nozzle: 15N1/min

Generation rate of internal and surface defects of cold-rolled steelplate: 2.1%

COMPARATIVE EXAMPLE 17-2

Immersion nozzle: two-hole nozzle

Intensity of static magnetic field: 0.3T

Flow rate of Ar gas injected in immersion nozzle: 15N1/min

Generation rate of internal and surface defects of cold-rolled steelplate: 1.6%

EXPERIMENTAL EXAMPLE 17-1

Immersion nozzle: single straight nozzle, discharged port (80 mmφ)

Set-up position of static magnetic field: apart from meniscus by900-1000 mm

Maximum intensity of static magnetic field: 0.3T, applying to wholewidth of casting, widthwise distribution of magnetic flux density; asshown in FIG. 25

Applied Current: 3000A (DC)

Generation rate of internal and surface defects of cold-rolled steelplate: 0.2%

WORKING EXAMPLE 18

FIG. 26 is a view for explaining the construction of this workingexample. A static magnetic generator 68 is disposed to a mold 10 at theposition corresponding to the meniscus. Further, directly under the mold10, there are provided static magnetic field generating coils 70 forgenerating a static magnetic field in the direction perpendicular to thelong side surface of the casting, and exciting rolls 72 for applying adirect current in the direction perpendicular to the short side surfaceof the casting. The static magnetic field generated at the staticmagnetic field generating coil 70 is applied to the whole width of thecasting 2 from a suitable point under the discharge port 20 of theimmersion nozzle, for example, at the position directly under the mold10. In FIG. 26, the directions of the magnetic field B, the current I,and the electromagnetic force F in the molten steel are shown in a chainline, a dashed line, and two-dot chain line, respectively.

Extremely low carbon aluminum-killed steel (C=15-25 ppm), which wasobtained by RH treatment after blowing in a converter, was continuouslycast by six strands (280t/strand) at a throughput of 5.2t/(min·strand)under the following condition.

Experimental condition

Size of slab: 230 mm (t)×1500 mm (W)

Type of continuous casting machine: vertical bending continuous castingmachine, two strands, vertical portion (3 m)

Superheat of molten steel in tundish: 15°-25° C.

Immersion depth of nozzle: 300 mm (distance between meniscus and nozzlejetting port)

Oxygen concentration of molten steel in tundish: 12-15 ppm

Length of mold: 900 mm

Distance between meniscus and lower end of mold: 800 mm

Slabs were continuously cast according to respective casting processesdescribed later, and then hot-rolled and cold-rolled to a thickness of0.4 mm. The cold-rolled steel plates thus obtained were examined in aninspecting line, and were compared with each other in the generationrate of sliver and blistering defects caused by steel-making. As aresult, according to the present invention, it is possible to extremelyreduce the generation rate of defects as compared with the conventionalcasting.

COMPARATIVE EXAMPLE 18-1

Immersion nozzle: two-hole nozzle, 75 mmφ×2, horizontal nozzle

Flow rate of Ar gas injected in immersion nozzle: 15N1/min

Generation rate of internal and surface defects of cold-rolled steelplate: 3.5%

COMPARATIVE EXAMPLE 18-2

Immersion nozzle: two-hole nozzle, 75 mmφ×2, horizontal nozzle

Intensity of static magnetic field: 0.3T, application of static magneticfield to only meniscus portion

Flow rate of Ar gas injected in immersion nozzle: 15N1/min

Generation rate of internal and surface defects of cold-rolled steelplate: 2.8%

WORKING EXAMPLE 18-1

Immersion nozzle: single straight nozzle, discharged port (85 mmφ)

Static magnetic field:

Meniscus portion: 0.2T, whole width of long side of casting, widthwisedistribution of magnetic flux density: uniform

Position apart from meniscus by 900-1000 mm, maximum intensity of staticmagnetic field: 0.3T, application to whole width of casting

Applied current: 2500A (DC)

Generation rate of internal and surface defects of cold-rolled steelplate: 0.1%

WORKING EXAMPLE 18-2

Immersion nozzle: single straight nozzle, discharged port (85 mmφ)

Static magnetic field:

Meniscus portion: not applied

Position apart from meniscus by 900-1000 mm: maximum intensity of staticmagnetic field: 0.4T, application to whole width of casting, widthwisedistribution of magnetic flux density; as shown in FIG. 27

Applied current: 2500A (DC)

Generation rate of internal and surface defects of cold-rolled steelplate: 0.6%

WORKING EXAMPLE 19

FIG. 28 is a view for explaining the construction of this workingexample 19. A static magnetic generator 74 is disposed to a mold 10 atthe position corresponding to the meniscus. Further, directly under themold 10, there are provided static magnetic field generating coils 76for generating a static magnetic field in the direction perpendicular tothe long side surface of the casting, and exciting rolls 80 for applyinga direct current in the direction perpendicular to the short sidesurface of the casting. The static magnetic field generated at thestatic magnetic field generating coils 70 is applied to the whole widthof the casting 2 from a suitable point under the discharge port 20 ofthe immersion nozzle, for example, at the position directly under themold 10. In FIG. 28, the directions of the magnetic field B, the currentI, and the electromagnetic force F in the molten steel are shown in achain line, a dashed line, and two-dot chain line, respectively.

Extremely low carbon aluminum-killed steel (C=15-25 ppm), which wasobtained by RH treatment after blowing in a converter, was continuouslycast by seven strands (310t/strand) at a throughput of 5.8t/(min·strand)under the following condition.

Experimental condition

Size of slab: 215 mm(t)×1500 mm(W)

Type of continuous casting machine: vertical bending continuous castingmachine, two strands, vertical portion (2m)

Superheat of molten steel in tundish: 18°-27° C.

Immersion depth of nozzle: 300 mm (distance between meniscus and nozzlejetting port)

Oxygen concentration of molten steel in tundish: 14-20 ppm

Length of mold: 900 mm

Distance between meniscus and lower end of mold: 800 mm

Slabs were continuously east according to respective casting processesdescribed later, and then hot-rolled and cold-rolled to a thickness of0.35 mm. The cold-rolled steel plates thus obtained were examined in aninspecting line, and were compared with each other in the generationrate of sliver and blistering defects caused by steel-making. As aresult, according to the present invention, it is possible to extremelyreduce the generation rate of defects as compared with the conventionalcasting.

COMPARATIVE EXAMPLE 19-1

Immersion nozzle: two-hole nozzle, 80 mm φ×2, horizontal nozzle

Flow rate of Ar gas injected in immersion nozzle: 15N1/min

Generation rate of internal and surface defects of cold-rolled steelplate: 4.5%

WORKING EXAMPLE 19-1

Immersion nozzle: two-hole nozzle, discharge port (90 mmφ×2)

Excitation of static magnetic field:

Meniscus portion: application of electromagnetic force downwardly ofcasting direction

Static magnetic field: 0.15T, whole width of long side of casting

Applied current: 1200A (DC)

Portion Directly under mold: application of electromagnetic forceupwardly of casting direction

Position apart from meniscus by 900-1000 mm:

Intensity of static magnetic field: 0.3T, application to whole width ofcasting

Applied current: 2800A (DC)

Generation rate of internal and surface defects of cold-rolled steelplate: 0.08%

WORKING EXAMPLE 19-2

The experiment was made in the same manner as in Working example 19-1,except that the excitation of the static magnetic field was not appliedto the meniscus portion.

Generation rate of internal and surface defects of cold-rolled steelplate: 1.8%

WORKING EXAMPLE 20

FIGS. 29 (a) and 29(b) show the construction of a main portion of acontinuous casting apparatus used in this working example. A staticmagnetic generator 82 is disposed on the back surface of long side wall14 of a continuous casting mold 10, and exciting terminals 84 areprovided for applying a direct current in the direction perpendicular tothe short side surface of the casting. In FIG. 29, the directions of themagnetic field B, the current I, and the electromagnetic force F in themolten steel are shown in a chain line, a dashed line, and two-dot chainline, respectively.

With this construction, according to the present invention, the staticmagnetic field generator 82 generates the static magnetic field in thedirection perpendicular to the long side surface of the casting in themolten steel within the mold, and simultaneously the exciting terminals84 apply the direct current in the direction perpendicular to the shortside surface of the casting, which makes it possible to form theelectromagnetic force upwardly of the casting direction. Therefore, itis possible to disperse the flow of the downward flow from the nozzle,and hence to suppress the permeation of the inclusions and babbles inthe casting.

Extremely low carbon aluminum-killed steel (C=15-20 ppm), which wasobtained by RH treatment after blowing in a converter, was continuouslycast by four strands (350t/strand) at a throughput of 4.5t/(min·strand)under the following condition.

Experimental condition

Size of slab: 240 mm (t)×1500 mm (W)

Type of continuous casting machine: vertical bending continuous castingmachine, vertical portion (2.5 m)

Superheat of molten steel in tundish: 15°-25° C.

Immersion depth of nozzle: 300 mm

Total oxygen amount in molten steel: 22-30 ppm

Injected amount of Ar gas: 5.0 N1/min

Conventional example: two-hole nozzle; static magnetic field, notapplied

Present invention: using straight nozzle

Excitation of static magnetic field: application of electromagneticforce upwardly of casting direction

Intensity of static magnetic field: 0.15T

Applied current: 1100A

The slabs thus continuously cast were hot-rolled and cold-rolled to athickness of 0.7 mm. The cold-rolled steel plates thus obtained weresubjected to continuous annealing, and then examined in an inspectingline, to be thus compared with each other in the generation rate of theoliver and blistering defects caused by steel-making. The generationrate of defects is represented by an equation of (weight of defectiveproducts)/(weight of inspected products)

Conventional example

Sliver: 0.02%

Blistering: 0.15%

Working example

Sliver: 0.03%

Blistering: 0.03%

In the sliver defects caused on the surface of the continuous casting bymold powder and alumina cluster, there is no difference between theconventional example and the working example. However, the generationrate of blistering defects in the working example is reduced to be 1/5as much as that in the conventional example. Accordingly, it becomesapparent that the working example is effective to suppress thepermeation of Ar gas injected from the nozzle and the inclusions withinthe casting.

Also, the casting test was made using the straight nozzle withoutexcitation of the static magnetic field, separately. However, in thiscasting condition, the jet of the high temperature molten steeldischarged from the leading edge of the nozzle was made to strongly flowin the vertical direction, and to wash the solidified shell, therebygenerating the breakout, which makes impossible the casting.

WORKING EXAMPLE 21

FIGS. 30 (a) and 29(b) show the construction of a main portion of acontinuous casting apparatus used in this working example. A staticmagnetic generator 86 is disposed on the back surface of a long sidewall 14 of a continuous casting mold 10. Also, exciting terminals 88 areembedded in refractories of the straight immersion nozzle 18 forapplying a direct current in the direction perpendicular to the shortside surface of the casting, thereby giving an electromagnetic force tothe molten steel in the direction of decelerating the flow of the moltensteel. In FIG. 30, the directions of the magnetic field B, the currentI, and the electromagnetic force F in the molten steel are shown in achain line, a dashed line, and two-dot chain line, respectively.

With this construction, according to the present invention, the staticmagnetic field generator 82 generates the static magnetic field in thedirection perpendicular to the long side surface of the casting in themolten steel within the mold, and simultaneously the exciting terminals84 apply the direct current in the vicinity of the nozzle discharge portin the direction perpendicular to the short side surface of the casting,which makes it possible to form the electromagnetic force upwardly ofthe casting direction. Therefore, it is possible to restrict anddisperse the flow of the downward flow from the nozzle, and hence tosuppress the permeation of the inclusions and babbles in the casting.

Extremely low carbon aluminum-killed steel (C=15-20 ppm) which wasobtained by RH treatment after blowing in a converter, was continuouslycast by four strands (350t/strand) at a throughput of 4.5t/(min·strand)under the following condition.

Experimental condition

Size of slab: 240 mm in thickness×1500 mm in width

Type of continuous casting machine: vertical bending continuous castingmachine, vertical portion (2.5 m)

Superheat of molten steel in tundish: 15°-25° C.

Immersion depth of nozzle: 300 mm

Total oxygen amount in molten steel: 25-30 ppm

Conventional example: two-hole nozzle; static magnetic field, notapplied

Working example: straight nozzle

Intensity of static magnetic field: 0.15T

Applied current: 1100A

Excitation of static magnetic field: application of electromagneticforce upwardly of casting direction

The slabs thus continuously cast were hot-rolled and cold-rolled to athickness of 0.7 mm. The cold-rolled steel plates thus obtained weresubjected to continuous annealing, and then examined in an inspectingline, to be thus compared with each other in the generation rate of thesliver defects and blistering defects caused by steel-making. Thegeneration rate of defects is represented by an equation of (weight ofdefective products)/(weight of inspected products)

Conventional example

Sliver: 0.02%

Blistering: 0.16%

Working example

Sliver: 0.03%

Blistering: 0.03%

In the sliver defect caused on the surface of the continuous casting bymold power and alumina cluster, there is no difference between theconventional example and the working example. However, the generationrate of blistering defects in the working example is reduced to be 1/5as much as that in the conventional example. Accordingly, it becomesapparent that the working example is effective to suppress thepermeation of Ar gas injected from the nozzle and the inclusions withinthe casting.

Also, the casting test was made using the a straight immersion nozzlewithout the excitation of the static magnetic field, separately.However, in this casting condition, the jet of the high temperaturemolten steel discharged from the leading edge of the nozzle was made tostrongly flow in the vertical direction, and to wash the solidifiedshell, thereby generating the breakout, which makes impossible thecasting.

WORKING EXAMPLE 22

The steel of the same kind as in Working example and containing a totaloxygen amount of 20 ppm or less was continuous cast under the samecondition as in Working example 21 except that Ar gas was not injectedin the immersion nozzle. The cold-rolled steel plates thus obtained wereexamined. In the steel plates continuously cast according to the presentinvention, rolled and annealed, there was obtained the preferableresults of sliver defects (0.01%) and blistering defects (0%). On thecontrary, in the conventional casting without gas injection, the desiredpouring speed was not achieved at third charge because of the nozzleblocking, and the casting speed was reduced from 1.6 m/min to 1.2 m/min.Needless to say, in the casting of the present invention, the castingspeed was not reduced, and only the alumina depositing layer of 1-2 mmand a slight blocking were recognized on the inner surface of thestraight nozzle after casting.

We claim:
 1. A process for continuously casting steel comprising thesteps of:supplying a direct flow of molten steel at a pouring speed ofat least 1.5 tons/minute from a tundish to a continuous casting moldthrough a straight immersion nozzle which has a single discharge port,while blowing an inert gas therethrough for preventing said nozzle fromclogging, said casting mold comprised of a pair of spaced long sidewalls interconnected to a pair of short side walls, a mold top, and amold bottom, wherein a vertical height between said mold top and bottomdefines a magnetic field operating range, said mold long side walls eachhaving a front surface and a back surface, an upper side and a lowerside, said immersion nozzle having a tube configuration with an upperand lower end, said lower end defining said discharge port; disposing arespective static magnetic generator on the back surfaces of the longside walls of said mold at a vertical region which includes the lowerend defining the discharge port of said straight immersion nozzle when amagnetic field is generated; and casting said molten steel whilegenerating a static magnetic field, said magnetic field directed fromone long side wall to the other long side wall of said mold in order tocontrol a direct flow rate of said molten steel into said mold.
 2. Aprocess for continuously casting molten steel as claimed in claim 1 inwhich when the static magnetic field generator applies a two-stagestatic magnetic field to the mold at a position lower than the level ofsaid discharge port, said magnetic field defined by a relationshipbetween a magnetic flux density B(T) and said magnetic field range L(mm)at various discharge flow velocities v(m/sec), said relationship beingset as follows:when v≦0.9 (m/sec), B×L≧16where B≧0.05T, L≧50 mm0.9≦v≦1.5 (m/sec), B×L≧18where B≧0.07T, L≧60 mm 1.5≦v≦2.0 (m/sec),B×L≧19where B≧0.08T, L≧70 mm 2.0≦v≦2.5 (m/sec), B×L≧20where B≧0.09T,L≧80 mm 2.5≦v≦3.0 (m/sec), B×L≧21where B≧0.1T, L≧90 mm 3.0≦v≦4.0(m/sec), B×L≧22where B≧0.11T, L≧100 mm 4.0≦v≦5.0 (m/sec), B×L≧24whereB≧0.12T, L≧100 mm 5.0≦v≦6.0 (m/sec), B×L≧26where B≧0.13T, L≧110 mm.
 3. Aprocess for continuously casting molten steel as claimed in claim 2 inthat an upper static magnetic field is applied over said entire width ofsaid mold.
 4. A process for continuously casting molten steel as claimedin claim 2 in that a lower static magnetic field is applied over saidentire width of said mold.
 5. A process for continuously casting steelcomprising the steps of:supplying a direct flow of molten steel at apouring speed of at least 1.5 tons/minute from a tundish to a continuouscasting mold through a straight immersion nozzle which has a singledischarge port, while blowing an inert gas therethrough for preventingsaid nozzle from clogging, said casting mold comprised of a pair ofspaced long side walls interconnected to a pair of short side walls, amold top, and a mold bottom, wherein a vertical height between said moldtop and bottom defines a magnetic field operating range, said mold longside walls each having a front surface and a back surface, an upper sideand a lower side, said immersion nozzle having a tube configuration withan upper and lower end, said lower end defining said discharge port;disposing a respective static magnetic generator on the back surfaces ofthe long side walls of said mold at a vertical region which includes thelower end defining the discharge port of said straight immersion nozzlewhen a magnetic field is generated; disposing a gap portion and furtherdisposing at least one additional stage of static magnetic fieldgenerators on the lower side of said mold, below said gap portion; andcasting said molten steel while generating a static magnetic field, saidmagnetic field directed from one long side wall to the other long sidewall of said mold in order to control a direct flow rate of said moltensteel into said mold.
 6. A process for continuously casting molten steelas claimed in claim 5 in that an upper static magnetic field is appliedover an entire region in a width direction of said mold.
 7. A processfor continuously casting molten steel as claimed in claim 5, in that alower static magnetic field is applied over an entire region in a widthdirection of said mold.
 8. A process for continuously casting steelcomprising the steps of:supplying a direct flow of molten steel at apouring speed of at least 1.5 tons/minute from a tundish to a continuouscasting mold through a straight immersion nozzle which has a singledischarge port, while blowing an inert gas therethrough for preventingsaid nozzle from clogging, said casting mold comprised of a pair ofspaced long side walls interconnected to a pair of short side walls, amold top, and a mold bottom, wherein a vertical height between said moldtop and bottom defines a magnetic field operating range, said mold longside walls each having a front surface and a back surface, an upper sideand a lower side, said immersion nozzle having a tube configuration withan upper and lower end, said lower end defining said discharge port;disposing a respective static magnetic generator on the back surfaces ofthe long side walls of said mold at a vertical region which includes thelower end defining the discharge port of said straight immersion nozzlewhen a magnetic field is generated; disposing a gap portion and furtherdisposing at least one additional stage of static magnetic fieldgenerators on the lower side of said mold, below said gap portion; andcasting said molten steel while generating a static magnetic field, saidmagnetic field directed from one long side wall to the other long sidewall of said mold in order to control a direct flow rate of said moltensteel into said mold.
 9. A process for continuously casting steelcomprising the steps of:supplying a direct flow of molten steel at apouring speed of at least 1.5 tons/minute from a tundish to a continuouscasting mold through a straight immersion nozzle which has a singledischarge port, while blowing an inert gas therethrough for preventingsaid nozzle from clogging, said casting mold comprised of a pair ofspaced long side walls interconnected to a pair of short side walls, amold top, and a mold bottom, wherein a vertical region between said moldtop and bottom defines a magnetic field operating range, said mold longside walls each having a front surface and a back surface and defining awidth of said mold, an upper side and a lower side, said immersionnozzle having a tube configuration with an upper and lower end, saidlower end defining said discharge port; disposing a respective staticmagnetic generator on the back surfaces of the long side walls of saidmold at a vertical region which includes the lower end of the dischargeport of said straight immersion nozzle when a magnetic field isgenerated; casting said molten steel while generating a static magneticfield, said magnetic field directed from one long side wall to the otherlong side wall of said mold in order to control a direct flow rate ofsaid molten steel into said mold; said magnetic field respectivelydefined by a relationship between a magnetic flux density B(T) andmagnetic field range L(mm) at various discharge flow velocitiesv(m/sec), said relationship being set as follows: when v≦0.9 (m/sec),B×L≧25where B≧0.07T, L≧80 mm
 0. 9≦v≦1.5 (m/sec), B×L≧27where B≧0.08T,L≧90 mm 1.5≦v≦2.0 (m/sec), B×L≧30where B≧0.09T, L≧100 mm 2.0≦v≦2.5(m/sec), B×L≧33where B≧0.09T, L≧110 mm 2.5≦v≦3.0 (m/sec), B×L≧35whereB≧0.1T, L≧110 mm 3.0≦v≦3.8 (m/sec), B×L≧36where B≧0.11T, L≧120 mm3.8≦v≦4.8 (m/sec), B×L≧38where B≧0.12T, L≧120 mm 4.8≦v≦5.5 (m/sec),B×L≧40where B≧0.13T, L≧130, wherein said static magnetic field isapplied over said entire width of said mold.